It is desirable to monitor the position of the armature within an electro-mechanical linear actuator to check that the device has either responded correctly to a command signal or that it has not been forced to an incorrect state by an external influence.
It is known to use a Hall effect magnetic sensor to monitor the armature position. This sensing technology can be compact and have a long lifetime whilst withstanding high operating temperatures.
However, a number of problems have been encountered with the use of Hall effect devices and other position sensors. As the sensor needs to be mounted on the actuator, this requires additional electrical wiring and connections between the actuator, sensor and control board. This increases the risk of equipment failure. In some applications, there may be insufficient physical space available to accommodate a position monitoring sensor. Furthermore, provision of a sensor may require an additional aperture in the actuator housing. This may make it more difficult to seal the actuator against ingress of fluids, particularly in high pressure environments.
Another potential problem is that the configurations of sensors can vary significantly and selection of a different sensor may require significant changes to the actuator configuration to accommodate it. A further drawback with Hall effect sensors is that they draw a significant amount of power. Thus it is preferable, particularly when relying on a local battery, to switch off the sensor when it is not in use, increasing the complexity of the control system. In addition, Hall effect sensors are not able to operate in very high temperature environments.